The group, called Anonymous, has been encouraging volunteers to download software called LOIC (Low Orbit Ion Cannon), which lets them centrally control these systems and direct them into a distributed denial of service attack (DDoS).

The point of the attacks is to put pressure on financial companies that recently cut ties with the WikiLeaks website over its planned publication of more than a quarter million US Department of State classified cables.

The Mastercard website went offline when about 400 computers targeted the LOIC software at it. By the time Visa was hit, however, there were more than 2,000 people using the software, according to Paul Mutton, a security analyst with Netcraft.

Visa was a much harder target to take down, because unlike Mastercard, it was distributed over the Akamai Technologies network. This meant that more Visa-hosting computers had to be overwhelmed to finally knock the site offline.